OCR F212 Enzymes

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Module 1: Biological Molecules
2.1.3 Enzymes
(a) state that enzymes are globular proteins, with a specific tertiary structure, which catalyse metabolic
reactions in living organisms
All enzymes are globular proteins with a specific tertiary
structure, which catalyse metabolic reactions in all living
organisms. This means that they speed up chemical reactions,
but are not 'usedup' as part of the reaction.
Enzymes are relatively large molecules, consisting of hundreds
of amino acids which are responsible for maintaining the specific
tertiary structure of the enzyme. Each enzyme has a specific
active site shape, maintained by the specific overall tertiary
structure. Therefore the tertiary structure must not be changed.
(b) state that enzyme action may be intracellular or extracellular
Extracellular enzyme action occurs outside the cell, which produces the protein. For example, some enzymes in
digestive systems are extracellular as they are released from the cells that make them, onto food within the
digestive system spaces.
Intracellular enzyme action occurs inside the cell, which produces the enzyme. For example, some enzymes in
digestive systems are found in the cytoplasm of cells or attached to cell membranes and the reaction takes place
inside the cell.
(c) describe, with the aid of diagrams, the mechanism of action of enzyme molecules, with reference to
specificity, active site, lock and key hypothesis, inducedfit hypothesis, enzymesubstrate complex,
enzymeproduct complex and lowering of activation energy.
Lock and Key Model Inducedfit Model
1. The shape of the enzyme's active site is 1. The substrate molecule collides with a
complementary and specific to the shape of the complementary and specific enzyme active site. The
substrate molecule the substrate is usually smaller. enzyme changes shape slightly to make the active site
2. The substrate collides with the active site so the fit more closely around the substrate. The substrate is
substrate 'key' fits into the active site 'lock' like a lock held due to oppositely charged groups on the substrate
and key. The substrate is then held in one place so the and the active site are found near each other.
reaction can go ahead, forming an enzymesubstrate 2. The enzymesubstrate complex is formed. The
complex. change in enzyme shape places a strain on the
3. The products are formed and it becomes the substrate molecule so that it destabilises it and the
enzymeproduct complex. The products formed are a reaction occurs more easily.
different shape from the substrate so they no longer fit 3. The products are made and it becomes the
into the active site so they move away. enzymeproduct complex. The products formed are a
different shape from the substrate so they no longer fit
into the active site so they move away.
The activation energy is the minimum level of energy required to enable a
reaction to take place. Enzymes work by lowering the activation energy of
reactions. This means reactions can proceed quickly at temperatures
much lower than boiling point as less energy is required for the reaction.

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Module 1: Biological Molecules
(d) describe and explain the effects of pH, temperature, enzyme concentration and substrate concentration on
enzyme activity
Effect Rate of Reaction Graph
pH At different pH's, the hydrogen ions interfere with
the hydrogen bonds and ionic bonds holding the
tertiary structure in place. Therefore the shape of
active site of an enzyme can change and so the
substrate will no longer fit.
All enzymes have their own optimum pH. This is the
pH at which the rate of reaction is highest.…read more

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Module 1: Biological Molecules
(e) describe how the effects of pH, temperature, enzyme concentration and substrate concentration on enzyme
activity can be investigated experimentally
Variable Method of Keeping Constant Reasons
pH Use buffer solutions that maintain pH at Reaction rate depends on the pH as if it is not at
a set level by keeping the H+ optimum pH, the tertiary structure can change
concentration in solution constant. and change the shape of the active site of the
enzyme.…read more

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Module 1: Biological Molecules
more likely than an inhibitor to collide with an active
site.
Reversible inhibitors are inhibitors that bind to the active site for a short period and then leave. The removal of the
inhibitor from the reacting mixture leaves the enzyme molecules unaffected.
Irreversible inhibitors are inhibitors that bind permanently to the enzyme molecule. Any enzyme molecules bound
by inhibitor molecules are effectively denatured.…read more

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Module 1: Biological Molecules
(h) state that metabolic poisons may be enzyme inhibitors, and describe the action of one named poison
Potassium Potassium cyanide inhibits cell respiration.
Cyanide It is a noncompetitive inhibitor for a vital respiratory enzyme called cytochrome
oxidase, found in mitochondria.
Inhibition of this enzyme decreases the use of oxygen, so ATP cannot be made and so
the organism must respire anaerobically, which leads to a build up of lactic acid in the
blood.…read more

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